The present invention relates to a cyan dye-donor element used in thermal transfer according to a thermal transfer-recording and a thermal transfer sheet using the same for color hard copies.
A method of printing image by thermal transfer, i.e., pictures are formed by causing dyes to sublimate or vaporize by heat, has come into the limelight recently as a method for obtaining color hard copies from televisions, CRT color displays, color facsimiles, magnetic cameras, and others. A thermal source in this method includes heating elements such as thermal head and since transfer amount of dye can be controlled according to thermal energy given, good continuous gradation color image can be obtained.
According to this method, sublimating or vaporizing dye coated on a substrate of thermal transfer sheet is transfer printed on an image receiving material by a thermal head controlled by image signal and full color images can be formed by using thermal transfer sheets having dyes of the three primary colors of yellow, magenta and cyan. Such thermal transfer sheet has been produced by selecting dyes having relatively good sublimatability or vaporizability and superior hue and fastness from various dyes such as disperse dyes and basic dyes (e.g., U.S. Pat. No. 4,695,287, Japanese Patent Kokai Nos. 60-239289, 61-268494, 61-268495, 62-64595 and European Patent No. 209991 (=Japanese Patent Kokai No. 63-15790).
Dyes used in thermal transfer sheet must satisfy various requirements as enumerated below and only when these requirements are satisfied, good image can be obtained.
(1) The dyes must have good solubility and dispersibility in resin or solvent component used in making thermal transfer sheet by coating a dye layer on a transfer substrate.
(2) The dyes must be easily diffused, sublimated or vaporized with heat onto an image receiving material (image printing layer) from a heat transfer sheet and have good affinity for resin of an image receiving material.
(3) The dyes must have optimum color characteristics, namely, hue, density and chroma as three primary colors for full color display in an image printing layer.
(4) The dyes must afford images excellent in fastness such as light resistance and migration resistance.
Various proposals have been made to satisfy the requirements for dyes and, for example, it has been proposed to use dyes having specific chemical structure or dyes having limited molecular weight and I/O value.
However, satisfactory dyes have not yet been obtained. Especially, cyan dyes have the defects that they are inferior in solubility in making thermal transfer sheet and they cannot give cyan color having desired hue.